1. Field of Invention
The present invention relates to selectively processing coal to remove and environmentally stabilize a substantial fraction of the mercury.
2. Background
The upgrading processing of coal can take a number of forms such as drying, pyrolysis and mild gasification. However in so processing little concern has been shown for where the heavy elements of environmental concern, in particular mercury, actually are deposited. Often during power plant operation they are conjectured to leave with the stack gases or remain in the ash. Sometimes the upgrading processing is alleged to remove them.
Recent governmental laws and regulations require an evaluation of the emissions of hazardous air pollutants, such as airborne mercury. Several studies, mandated by law, are scheduled for the near future and among these are evaluations of mercury emissions on human health and the environment. Therefore, the ability to reduce mercury emissions will be paramount in the near future. In the long range future all mercury releases of any manner may become a concern.
One study showed that mercury from coal ended up primarily in the flue gases; Brown and Schmidt, "Characterization of Hazardous Air Pollutants from Coal-Fired Electric Utilities," ACS National Meeting, Denver, March 1993, hereinafter Brown (1993). When coal is preprocessed before being sent to utilities such as has been proposed for low-sulfur Western coals, the study is likely unapplicable since mercury is removed unknowingly during processing.
The processing of coal, especially Western coal, for power plants starts with drying. Coal is dried for a variety of reasons, such as to save on transportation costs, to increase the heating value, to increase the net dollar value, to prevent handling problems caused by freezing weather, to improve coal quality particularly when used for coking, briquetting, and producing chemicals, to improve operating efficiency and reduce maintenance of boilers, and to increase coke oven capacity. However drying of coal causes increased dust formation as the dry coal is more friable. Further readsorption of moisture of dried coals is often considered a potential problem. In all this processing where the mercury originally present in mined coal becomes deposited is unknown.
The general problem of coal drying represents removing three types of moisture: free, physically bound, and chemically bound. Free moisture is found in the very large pores and interstitial spaces of coal and often is removed by mechanical means as it exhibits the normal vapor pressure expected of water at that temperature.
Physically bound moisture is more difficult to remove as it is held tightly in small coal capillaries and pores. Because of this, its vapor pressure and specific heat are reduced over that expected of free moisture.
Chemically bound moisture is characterized by a bonding between surfaces and water. Monolayer and multilayer bonding are commonly identified.
Sometimes a fourth type of moisture is identified which comes from the decomposition of organic compounds. It is really not moisture held in coal but is produced during coal decomposition.
Coal drying is characterized by typical drying curves that exhibit distinct rate regions. Firstly, a transient region occurs as equilibrium conditions are sought while the material heats. This is followed by a largely constant rate portion of drying where the material temperature is relatively constant during the unbound moisture removal, and the drying rate is generally determined from only the particle size and moisture content, be it coal or some other material.
The final region is a period of decreasing rate as the material temperature increases and the physically and chemically bound moisture is removed. For this drying regime the particle size, temperature, and residence time are important parameters. Often the drying rate becomes diffusion controlled, and since diffusivity increases with temperature, higher temperatures are employed to continue drying the materials.
During the constant rate period, the heat and mass transfer rates are directly proportional to the driving forces of temperature gradient and humidity gradient respectively; the appropriate proportionality constants, however, are usually experimentally determined. Maintaining large values of said gradients become important when efficient drying equipment is designed; however, if drying residence time is increased easily, such gradients become less important. On the other hand when the concern is vaporization of mercury metal, temperature gradients can also effect its rate.
For many coals with higher moisture content, the most important variable is often the degree of fines produced for higher velocity drying gases entrain more such fines.
Equipment to control particulate emissions, especially from fluidized bed dryers, includes combinations of cyclones, electrostatic precipitators, bag filters, and wet scrubbers. Cyclones are ineffective with particle sizes below five microns, so their operation is usually restricted to extraction of large particle dust loading prior to removal of fine dust particles by subsequent equipment. However cyclones employed at the gas stream dew point or with water-spraying, are nearly as effective as wet scrubbers. Electrostatic precipitators operate free of condensation, and in addition, are subject to malfunctions and frequent maintenance. When superimposing the mercury problem on this equipment, consideration of whether mercury in some vapor form is adsorbed, or maybe absorbed, onto coal particle dust fines. One study has shown that mercury vapor not associated with dust particles generally passes through filters and electrostatic precipitators; see Brown (1993). Another study concludes that most mercury behaves as a vapor even in the presence of particulate matter; see Otani et al., "Adsorption of Mercury Vapor on Particles," 20 Environmental Science Technology, 735, 1986.
Often such dust after collection is returned to the processed coal in some manner. Under this circumstance mercury present with such fines would have been rearranged but not removed. Sometimes such fines are burned for the energy requirements of the process, and in this case, any mercury might end up in ash or stack gases. In general it will remain in the locality of the coal drying operation in contrast to the power plant region. Of course during mine upgrading processing, some unvaporized mercury remains in the processed coal and is transported to the power plant location.
At temperatures higher than that employed for drying, pyrolysis of coal occurs, and this takes many forms often concentrating on the various products of mild gas, hydrocarbon liquids and solid char. Whereas previously much pyrolysis design stressed obtaining maximum yields of liquid and gaseous products, modern operations now concentrate upon well-controlled partial pyrolysis designed to produce selected outputs that are recycled within the process to make the final processed coal product.
Prior art United States patents covering the above mentioned coal processing to isolate mercury include:
______________________________________ U.S. Pat. No. Inventor Year ______________________________________ 3,876,393 Kasai et al 1975 4,101,631 Ambrosini et al 1978 4,491,609 Degel et al 1985 4,892,567 Yan 1990 4,986,898 Nisimura et al 1991 ______________________________________
Referring to the above list, Kasai et al disclose removing mercury from gases by employing activated carbon impregnated with sulfuric acid solution. Ambrosini et al disclose removing mercury vapor from gas streams by using crystalline zeolitic molecular sieves. Degel et al disclose producing carbonaceous adsorbents impregnated with elementary sulfur. Yan discloses simultaneously removing mercury and water with molecular sieves comprising silver or gold on zeolite. Nisimura et al disclose removing mercury from hydrocarbon oil special treating agent comprising some metals or their selected inorganic compounds.